The present invention generally relates to an electrical mixer and more particularly, to a double-balanced mixer for use, for example,, in a CATV (cable television) tuner or the like.
In FIG. 4, there is shown one example of a conventional double-balanced mixer of the above type which includes four diodes 11, 12, 13 and 14 connected effectively in a bridge connection between a transmission line transformer 22 as a phase distributor and phase distributors 31 and 32 composed of coils and capacitors, with input and output transmission line transformers 21 and 23 being provided if needed.
High frequency signals RF supplied to a terminal 41 are subjected to frequency conversion by the transmission line transformer 22, the diodes 11 to 14 and the phase distributors 31 and 32, based on a local oscillator signal LO fed to a terminal 42, and thus, intermediate signals IF are outputted from another terminal 43.
The characteristics of the diodes 11, 12, 13 and 14 may be represented by X1, X2, X3 and X4 (such characteristics mainly being related to the capacitance across its terminals and the forward direction voltage-current characteristics) and should be in a relationship as in EQU X1=X2=X3=X4
in order to maintain a balancing of the circuit, and therefore, it has been a conventional practice to select diodes having uniform characteristics from among many diodes so as to use four diodes as one set, thus requiring a troublesome procedure for the selection of diodes.
Moreover, when the respective diodes 11 to 14 are to be actually attached onto a printed circuit board 50, in order to maintain, the balanced inductance thereof by making the length of lead wires of the respective diodes 11 to 14 equal, it is necessary to dispose one of the intersecting diodes (the diode 12 in the drawing) on a reverse surface of the printed circuit board 50 as shown in FIGS. 5(A) and 5(B), and thus, more labor is required for the attaching and soldering of the diodes to be mounted on both surfaces of the circuit board 50.
Furthermore, in the case where the diodes 11 to 14 are to be attached onto both surfaces of the printed circuit board as described above, soldering thereof must be effected two times separately for the front and reverse surfaces of the printed circuit board 50, and at this time, since the selected four diodes are required to be used for one set as described earlier, management of diodes has been necessary so that one set of diodes is not confused with another set of diodes during the two soldering operations, thus also requiring a troublesome procedure therefor.